Inge Lehmann and the rise of international seismology, 1925-1970

Danish seismologist Inge Lehmann (1888-1993) is best known for presenting the first evidence of the Earth’s inner core in 1936. Her active research career as a seismologist began in 1928, continued well into the 1970s, and earned her the reputation as “the grande dame” of modern seismology.

By PhD A. Lif Lund Jacobsen. Archivist and researcher at The Danish National Archives

Danish seismologist Inge Lehmann (1888-1993) is best known for presenting the first evidence of the Earth’s inner core in 1936. Her active research career as a seismologist began in 1928, continued well into the 1970s, and earned her the reputation as “the grande dame” of modern seismology.

Born in a time when few women could hold senior scientific positions, Lehmann had an extraordinary career: In 1928, she was appointed Head of the Seismic Section at the Danish Geodetic Institute, where she published evidence for the existence of the Earth’s inner core in 1936. After her retirement in 1953, she continued her work at research institutions in the USA, and in 1964, she proved a velocity discontinuity at a depth of 200 km, known today as the “Lehmann Discontinuity”.

Inge Lehmann’s private archive at the Danish National Archive (Rigsarkivet) contains more than 4,000 letters and documents relating to her research. As an important scientist in her field, she corresponded with many of the leading seismologists of the time. Few of her private letters have survived.

During Inge Lehmann’s lifetime, seismology, the study of earthquakes and propagation of elastic waves through the Earth, developed from a small, isolated discipline to a large, well-funded research area. This growth took place against the backdrop of the Cold War with its political and military agendas. Seismology attracted special interest because it provided tools for the detection of nuclear weapons tests.

Inge Lehmann’s career can therefore be contextualised into a wider framework of discipline development, international politics, and gender studies. By taking a biographical approach to her life and career, in the form of a book, it is possible to reveal the much larger story of how international seismology developed from a small, obscure discipline to a science with powerful geopolitical implications.

This project will provide the first in-depth account of the beginning and development of modern international seismology, and the role of Cold War politics in it.

Inge Lehmann signature. Credit: Robert Wengler

Experiences as an early female scientist

She was born on 15 May 1888 in Østerbro by Ida Mee Tøfslev and Alfred Lehmann. Her father and great inspiration was the first professor in psychology at Copenhagen University, and her mother’s side of the family counted several prominent members of the Women’s Liberation Movement.

Inge Lehmann has often been portrayed as a trailblazing female scientist, unwilling to accept discrimination in her pursuit of an academic career. However, her personal archive at the Danish National Archive shows that Inge Lehmann had to accept restrictions in her academic career due to her gender, and, only by being pragmatic about her situation, did she succeed in establishing herself as a professional scientist.

Having attended a co-educated public school and proceeded to study mathematics at Copenhagen University in 1907, Inge Lehmann had little first-hand experience with gender discrimination. After receiving her bachelor’s degree, in 1911, she went to study at Cambridge University, where she found herself unprepared for the social segregation of genders practiced there. To her surprise, female students could only attend lectures, and not use the university’s libraries or laboratories. When she went to meet her friend Niels Bohr, she had to be accompanied by a chaperone. While studying for the Mathematical Tripos, Inge Lehmann exhausted herself, and had to return to Denmark, where she abolished her studies altogether. Needing an income, she began to work in an insurance office, but she soon realized that her career options were severely limited by her gender. Consequently, in 1918, Inge Lehman returned to Copenhagen University to complete her studies. After graduating in 1923, she became a teaching assistant to the professor of actuary science. Realizing that her options for getting a scientific position at the university were small, she instead joined Professor Niels Erik Nørlund in his attempt to reform the government’s Geodetic Service.

At that time, there was a growing international pressure for Denmark to contribute seismological data to the international scientific community, and data from seismological stations in Greenland were especially sought after. By advocating Denmark’s need to express its sovereignty in Greenland for the international community, Erik Nørlund was able to gain the necessary political and financial support for establishing a network of seismological stations in Denmark and Greenland. In 1928, he established a permanent Danish seismological monitoring authority under the newly founded Danish Geodetic Institute.

In return for changing her field of study from mathematics to seismology, Nørlund agreed to promote Inge Lehmann to chief of the Seismological Section. It was a new position without much prestige, and with plenty of administrative work, but a scientific position never the less.

As an early internationally recognised female scientist, Lehmann personifies the structural challenges and personal costs experienced by early female scientists.

Finding the Earth’s Inner Core

As the chief of the Seismic Section, Inge Lehmann’s duties were to organize the work at the seismic stations in Copenhagen, Ivittuut and Ittoqqortootmitt (Scoresbysund), analyse all the seismograms to determine the arrival times of seismic waves, and write them up so they could be published in internationally circulated annual bulletins. These lists of arrival times were the basis upon which many early seismologists did their research.

As Inge Lehmann did all the readings from all three stations, her work gave her great practice in interpreting and comparing seismograms. In a time, where standardization of instruments and practices was still impossible, her ability to visually “calibrate” data from different seismograms made her attuned to observations and structures, which others might miss.

At the time, seismologists discussed the basic layout of the Earth’s interior. Based upon the arrival times of P-waves in the internationally published bulletins, seismologists tried to map the paths of seismic waves through the Earth’s mantle and core, working on a two-layered Earth model. By reading seismograms, Inge Lehmann became aware that a variation in the P-wave showed up in what was known as the Earth’s shadow zone, where P-waves were believed to be absent due to them being reflected off the boundary of the core. She theorized that the existence of this variation in the P-wave, which she called P’, could only be possible if the wave was reflected off a previously unknown inner core with a higher seismic wave speed. (1) She published her hypothesis in 1936, and, within a few years, her three-layered Earth model was generally accepted.

Cold War activities

Early in the Cold War, it was discovered that nuclear explosions showed up on seismic instrumental recordings just like earthquakes, and that seismic monitoring could be used remotely to detect and analyse (secret) nuclear explosions. Soon, seismic detection of underground nuclear explosions became a matter of national and military strategic interest, and funding for seismic research increased dramatically. In the USA, the military became the primary source of funding. (2)

In the USA, the large VELA UNIFORM-program (1963-1971) was designed to improve the “state of the art” of seismology, and develop better methods of detecting underground nuclear explosions.

In Denmark, Inge Lehmann was fed up with administrative work and contemplating retirement, when she, in 1952, was approached by Maurice Ewing from Columbia University, who suggested that she join his seismic research team at Lamont Geological Observatory. The visit became a turning point in Inge Lehmann’s career, and, the year after, in 1953, she took early retirement from her position at the Danish Geodetic Institute to focus on her research. During the following 20 years, she spent months at a time in the USA, often working on research projects associated with nuclear detection, funded by military contracts. Her research into discontinuities in the Earth’s upper mantle depended upon seismic data from US nuclear tests conducted under VELA UNIFORM’s auspices, and, in 1964, she presented evidence for a velocity discontinuity at a depth of 200 km. (3) Today, the discontinuity bears Lehmann’s name.

In the USA, Inge Lehmann found a vibrant and well-funded research environment, which in turn appreciated her expertise and international connections. It was while working in the USA that she began receiving scholarly awards and honours, but her fame was largely ignored in Denmark.

A USA operated seismic test-station on Greenland’s ice cap was in 1966 named in her honour. Image credit Robert Wengler

The value of funding from the Carlsberg Foundation

Generous funding from the Carlsberg Foundation has allowed me a rare opportunity to do extensive and in-depth research in more than 20 historical archives and collections relating to Inge Lehmann and the history of early seismology abroad and in Denmark. As much of this material has never been open for research before, the projects have made a significant contribution to identify previously unknown historical material about Inge Lehmann and early seismology.

To have one’s research project supported by the Carlsberg Foundation with a postdoctoral fellowship has been invaluable in terms of scholarly acknowledgement, and has advanced my academic career.

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